The role of calcium in mammalian physiology is one of the most active areas of current biomedical research, with over 7,500 papers a year published on the subject. Despite the wealth of information concerning the control of calcium metabolism, scant attention has been paid to the role of calcium intake in calcium homeostasis. There are several clear demonstrations that manipulations of calcium metabolism influence the motivation to drink calcium but there has been no concerted attempt to study the physiological substrate of this behavior. The absence of these data is a critical omission in light of the low intake of calcium by most Americans, and the involvement of calcium in many chronic diseases (e.g. osteoporosis, kidney disease, heart disease, hypertension). The studies proposed here are designed to elucidate the physiological processes underlying the motivation to drink calcium. The questions addressed include (1) "Which aspects of calcium metabolism initiate calcium appetite?" To answer this question, calcium metabolism will be manipulated by (a) depriving rats of dietary calcium, and (b) administering calcium-regulatory hormones. The effect of these treatments on calcium intake and calcium metabolism will be compared. Measures will be taken of calcium intake, calcium balance, blood concentrations of calcium, parathyroid hormone (PTH), calcitonin, and I ,25-dihydroxyvitamin D, bone reabsorption, and tissue calcium content. The goal will be to determine which of these facets of calcium metabolism is responsible for the control of calcium appetite. The second question addressed is (2) "How does ingested calcium satiate calcium appetite?". Studies designed to answer this will (a) relate changes in calcium physiology to the satiation of calcium appetite, and (b) localize the organs involved. These studies will provide a solid foundation for understanding the physiological controls of calcium intake, and the role of calcium intake in calcium homeostasis. This information will help identify the reasons for the low intake of calcium by Americans, and provide strategies to help rectify this problem.